The portion of a tire comprising the bead core, apex and other reinforcing rubber strips form the tire bead and are designed so as to anchor the tire on the rim. The bead core is substantially inextensible in the circumferential direction and can be built up of a single or several elements, such as steel wire or cords. The diameter of the radially innermost surface of the bead core or bead core base corresponds roughly to the diameter of the rim bead seats except for the width of the carcass ply, chafer, apex and possibly other thin rubber linings. The tire's bead diameter is smaller than the rim's diameter, thereby yielding an interference fit when the tire is mounted onto the rim. Contact pressure of the tire on the rim depends on the geometrical relationship between the tire's bead area and the rim, the construction and position of the bead core, the inflation pressure, and other tire loads such as vertical, lateral, and centrifugal forces. The larger the contact pressure, the tighter the fit of the tire on the rim. If rim slip is of concern, one may consider reducing the tire's diameter to make the fit tighter. However, with the increased interference, the required inflation pressure to mount the tire may become so high as to impose danger to the operator.
Due to the rigidity of the bead cores, mounting of the tires on the rim requires deformation of the beads from a circular to an oval shape and application of forces approaching limits of elastic deformability of the steel wires or cords. Reducing the section of the bead and more specifically the section of the bead core would facilitate the mounting of the tire on the rim. However reducing the material in the bead will also work against a stable seat of the tire on the rim Under high inflation pressures the bead will have a tendency to jump over the rim flange. Standard burst resistance tests, such as inflating the tire under a (water) pressure of about 450 psi (3,000 kPa), will no more be met. The tire will either bounce over the rim flange or the textile reinforcements in the bead area will break.
U.S. Pat. No. 5,263,526 discloses a pneumatic tire having a specified bead structure allowing an improved seat of the bead on the rim. The cross-section of the bead core is roughly triangular and the width of the bead heel surface is approximately equal to the distance between a hump and an axially inward surface of the wheel flange.
U.S. Pat. No. 4,580,610 discloses a tire bead seat having taper angle which is greater than that of the rim bead seat such that on fitment of the tire to the rim the annular bead member and the associated edge of the carcass ply rotate around the bead core and cause the carcass ply to be placed under tension. In one embodiment the taper angle may be around 30°.
In U.S. Pat. No. 5,464,051 to Beard et al, a specified bead seat profile for passenger tires was proposed whereby the bead seat had a single taper angle of about 6.5° for a rim with a ledge taper of 5°. It was claimed for a cylindrical strap bead core with a flat base that the uniform compression of the rubber gauge under the bead core reduced the variability in the mounting pressure. Beard's comparative control tire of the prior art had an angular bead seat profile having a 5° and a 10.5° taper angle combination. The 5° to 10.5° transition point was close to the heel.
The present invention departs from the prior art approaches in that the focus is placed on the interaction of the bead core base and the angular orientation of the bead seat profile.
A first object of the invention is to reduce bead deformation and more specifically to improve toe lift properties of tires during their lifetime. A further object is to improve mountability of tires on the rims. A still further object is to reduce the tires' weight without diminishing their burst resistance.